Firm says changes resulted in unintentional nuke plant wear

The manufacturer of steam generators blamed for the yearlong shutdown at the San Onofre nuclear plant made its first public response Thursday, acknowledging that its manufacturing and design practices resulted in unprecedented damage to the equipment.

Hitoshi Kaguchi, a nuclear projects director at Mitsubishi Heavy Industries, told the five members of the Nuclear Regulatory Commission how alterations to anti-vibration tube supports unintentionally led to a form of destructive tube-to-tube wear never seen before in the industry before.

At a commission meeting in Rockville, Md., Kaguchi endorsed a restart plan for San Onofre’s Unit 2 reactor developed by Edison and vetted by outside consultants. He acknowledged that rapid degradation of the generators was a consequence of design elements that changed the speed and consistency of steam flows, without going into great detail.

The power plant’s two reactors have been offline since Jan. 31, 2012, when a steam generator tube leak generated a small radiation release. Inspections uncovered unprecedented wear on tubing that acts as a barrier to radiation and keeps coolant circulating through the reactor core.

San Onofre nuclear plant operator Southern California Edison on Thursday rebutted claims by U.S. Sen. Barbara Boxer, saying it would never install steam generators knowing that they would not perform safely.

In a statement, Edison called Boxer’s suggestion that Edison and generator manufacturer Mitsubishi Heavy Industries knew in advance about design issues “simply not accurate.”

Edison “would not, and did not, install steam generators that it believed would not perform safely,” the Rosemead-based utility said.

Boxer contends that Mitsubishi and Edison were aware of safety problems with steam generators before the equipment was installed starting in 2009 but rejected some enhancements to avoid a more rigorous regulatory review. The Democratic senator from California cited information contained in a 2012 report by Mitsubishi about the root causes of tube wear on its replacement generators at San Onofre.

The full report has not been released. Mitsubishi, Edison and the nuclear commission say the analysis contains proprietary information. Boxer and U.S. Rep. Ed Markey, D-Mass., have urged Nuclear Regulatory Commission Chairman Allison Macfarlane to investigate.

The exchange did little to resolve questions about whether Edison appropriately tailored its steam generator replacement project to regulatory procedures or sought to sidestep safety requirements.

Edison said it “sought to purchase replacement steam generators that would meet or improve upon the safety standards and performance of the original steam generators.”

Nuclear safety activists say changes to steam generators were extensive enough to warrant a more thorough license-amendment review by the regulatory commission that might have caught design flaws.

Edison and Mitsubishi engineers have described in a trade magazine how they tailored a “like-for-like” replacement to move forward without prior approval.

Design changes included the addition of about 400 tubes, the removal of a central support cylinder and a new format for tube support plates and anti-vibration bars intended to prevent tube wear.

The most significant damage, Edison and regulators say, has come from tubes rubbing against each other as never before.

The tube-to-tube wear has been traced to Mitsubishi computer models that miscalculated steam flows within the redesigned generators.